The present disclosure relates generally to renewable energy systems such as photovoltaic power systems, wind power systems, hydroelectric power systems, and other power systems that generate power using renewable energy sources. More specifically, the present description relates to systems and methods for controlling ramp rate and frequency regulation in a renewable energy system.
Increased concerns about environmental issues such as global warming have prompted an increased interest in alternate clean and renewable sources of energy. Such sources include solar and wind power. One method for harvesting solar energy is using a photovoltaic (PV) field which provides power to an energy grid supplying regional power.
Availability of solar power depends on the time of day (sunrise and sunsets) and weather variables such as cloud cover. The power output of a PV field can be intermittent and may vary abruptly throughout the course of a day. For example, a down-ramp (i.e., a negative change) in PV output power may occur when a cloud passes over a PV field. An up-ramp (i.e., a positive change) in PV output power may occur at sunrise and at any time during the day when a cloudy sky above the PV field clears up. This intermittency in PV power output presents a problem to the stability of the energy grid. In order to address the intermittency of PV output power, ramp rate control is often used to maintain the stability of the grid.
Ramp rate control is the process of offsetting PV ramp rates that fall outside of compliance limits determined by the electric power authority overseeing the grid. Ramp rate control typically requires the use of an energy source that allows for offsetting ramp rates by either supplying additional power to the grid or consuming more power from the grid. Stationary battery technology can be used for such applications. Stationary battery technology can also be used for frequency regulation, which is the process of maintaining the grid frequency at a desired value (e.g. 60 Hz in the United States) by adding or removing energy from the grid as needed. However, it is difficult and challenging to implement both ramp rate control and frequency regulation simultaneously.
Ramp rate control and frequency regulation both impact the rate at which energy is provided to or removed from the energy grid. However, ramp rate control and frequency regulation often have conflicting objectives (i.e., controlling PV ramp rates vs. regulating grid frequency) which increases the difficulty of using the same battery for ramp rate control and frequency regulation simultaneously. Additionally, conventional ramp rate control and frequency regulation techniques can result in premature degradation of battery assets and often fail to maintain the state-of-charge of the battery within an acceptable range. It would be desirable to provide solutions to these and other disadvantages of conventional ramp rate control and frequency regulation techniques.